An electric power steering apparatus, which urges a steering apparatus of an automobile or a vehicle with an assist load using rotational force of a motor, auxiliarily applies driving force of the motor to a steering shaft or a rack axis through a transmission mechanism such as gears or belts via a reduction gear.
Such a conventional electric power steering apparatus executes a feedback control of a motor current for accurately generating an assist torque (a steering assist torque).
The feed back control is to adjust a motor impressed voltage so as to reduce a difference between a current command value and a motor current detection value. The motor impressed voltage is generally adjusted based on a duty ratio of pulse width modulation (PWM) control.
A general structure of the electric power steering apparatus will be explained below with reference to FIG. 9. A shaft 2 of a steering wheel 1 is connected to a tie rod 6 of running wheels through a reduction gear 3, universal joints 4a and 4b, and a pinion rack mechanism 5. The shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1. A motor 20 for assisting the steering force of the steering wheel 1 is connected to the shaft 2 through a clutch 21 and the reduction gear 3. A control unit 30 for controlling the power steering apparatus is supplied with power from a battery 14 through an ignition key 11 and a relay 13. The control unit 30 calculates a steering auxiliary command value I of an assist command based on a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12. The control unit 30 then controls a current to be supplied to the motor 20 based on the calculated steering auxiliary command value I. The clutch 21 is turned on and off by the control unit 30 and is turned on (coupled) in an ordinary operating state. When it is determined by the control unit 30 that the power steering apparatus fails and when the power (voltage Vb) of the battery 14 is turned off by the ignition key 11 and the relay 13, the clutch 21 is turned of (disconnected).
The control unit 30 is mainly composed of a CPU. FIG. 10 shows general functions to be executed based on a program inside the CPU. For example, a phase compensator 31 does not show a phase compensator as independent hardware and shows a phase compensation function executed by the CPU. Functions and operation of the control unit 30 will be explained below. A steering torque T detected by the torque sensor 10 and then input is phase-compensated by the phase compensator 31 for increasing the stability of the steering system. Phase-compensated steering torque TA is input to a steering auxiliary command value calculator 32. The vehicle speed V detected by the vehicle speed sensor 12 is also input to the steering auxiliary command value calculator 32. The steering auxiliary command value calculator 32 determines a steering auxiliary command value I as a control target value of a current to be supplied to the motor 20, based on the input steering torque TA and the input vehicle speed V. The steering auxiliary command value I is input to a subtracter 30A, and is also input to a differential compensator 34 of a feed-forward system for increasing a response speed. A difference (I-i) calculated by the subtracter 30A is input to a proportional calculator 35, and a proportional output from the proportional calculator 35 is input to an adder 30B and is also input to an integration calculator 36 for improving the characteristic of a feedback system. Outputs from the differential compensator 34 and the integration calculator 36 are also input to and added together by the adder 30B. A result of the addition by the adder 30B is obtained as a current command value E, and this is input to a motor driving circuit 37 as a motor driving signal. A motor current value i of the motor 20 is detected by a motor current detection circuit 38, and this motor current value i is input to the subtracter 30A and is fed back.
When, however, the motor current detection circuit 38 fails, an accurate motor current cannot be measured. As a result, a disadvantage arises in that an excessive current flows to the motor and an excessive assist torque is generated thereby or a necessary amount of a current does not flow to the motor and a sufficient assist torque cannot be obtained.
Further, when a current is caused to flow to the motor to confirm operation of the motor current detection circuit 38, the motor is rotated and the steering wheel 1 is turned when the clutch 21 is connected, from which an unexpected accident may occur.
Accordingly, it is a very important problem to provide a means for determining a failure of the motor current detection circuit to prevent an accident. Heretofore, the inventors of the present invention have made a proposal for solving the above problem (Jpn. Pat. Appln. KOKAI Publication No. 8-91240).
The conventional motor current detection circuit is arranged such that a voltage, which is generated by a current flowing through a current detection resistor, is peak-held so as to amplify the signal thereof, and a current of a motor is controlled and a failure of the motor is detected using the signal.
Abnormality of the conventional current detection circuit is detected for the purpose of detecting an abnormal drift voltage of the current detection circuit caused by a failure just after a steering apparatus is shipped (initial diagnosis). In the detection of the abnormality, a value of a drift voltage, which is previously stored (at the time of shipment), when a current is detected, is compared with a value of a drift voltage detected immediately after an ignition switch is turned on, and it is diagnosed whether a difference between the former value and the latter value is more increased or more reduced than a predetermined value (initial diagnosis).
Further, in order to detect a failure of the current detection circuit due to short-circuit occurred therein, a current is caused to flow to the motor for a period of time longer than an electric time constant of the motor and shorter than a mechanical time constant thereof, and it is confirmed whether or not the current flows in an amount larger than a predetermined current value (Jpn. Pat. Appln. KOKAI Publication No. 8-91240).
Further, in order to enhance controllability and safety, the inventors have proposed to use two MCUs, i.e. a main MCU and a sub MCU and to input the current detection signal to the main MCU and to the sub MCU by branching it thereto so that a current is controlled and a failure is detected as shown in FIG. 11 (Jpn. Pat. Appln. KOKAI Publication No. 2001-18819).
More specifically, in FIG. 11, a voltage, which is generated by causing a motor current i to flow through a current detecting resistor (shunt resistor), is input to a motor current detection circuit 112 and input to the main MCU 101 and the sub MCU 102 passing through a low-pass filter. Accordingly, when one of the MCUs fails, the voltage input thereto is different from that input to the other MCU, which permits a failure of the MCUs to be found easily, thereby the controllability and the safety can be enhanced.
When it is determined by the abnormality detection means as described above that the current detection circuit fails, motor current control is interrupted, and the steering apparatus is placed in an assist stop state.
However, even if the controllability and the safety are enhanced by providing the two MCUs as described above, there remain three problems as described below.
First, a failure of a current amplifier of the motor current detection circuit cannot be accurately detected.
That is, a failure of the current amplifier of the motor current detection circuit cannot be detected, in particular, the failure cannot be detected when it occurs while a steering operation is being executed. If the current amplifier fails while the steering operation is being executed and a motor detection current becomes smaller than a normal value, it is determined by the current control executed in the MCUs that the motor detection current is smaller than a current command value, and a current larger than a predetermined current value flows to the motor. When a driver steers a vehicle in the above state, a torque becomes larger than an estimated assist torque, from which a possibility arises in that a steering wheel is excessively jerked or placed in an unstable state accompanied with vibration.
To cope with the above problem, there is required an abnormality detection means for determining whether or not the current amplifier of the motor current detection circuit is normal or not.
Second, a failure of the motor current detection circuit cannot be detected when a motor current is controlled (when motor current control is executed).
While the motor current control is being executed, it is executed by making a current control calculation by the MCUs. Accordingly, in order to detect the failure of the motor current detection circuit, a failure thereof due to offset and short-circuit must be detected by interrupting the motor current control calculation once.
Accordingly, abnormality of the motor current detection circuit cannot be detected by diagnosis other than the initial diagnosis.
However, since electronic parts may fail accidentally, it is necessary to diagnose a failure of the motor current detection circuit while it is in operation.
Third, the assist must be interrupted when the motor current detection circuit fails, which means that any alternate control means is not provided.
Conventionally, when it is diagnosed in the initial diagnosis that the motor current detection circuit fails, an assist control must be stopped thereafter because there is no means for normally detecting a motor current.
An object of the present invention, which was made to solve the above problems, is to provide a controller for an electric power steering apparatus which can accurately detect a failure of a current amplifier of a motor current detection circuit, has a function capable of detecting abnormality of the motor current detection circuit even while a vehicle is being steered, and can execute alternate control when a failure is detected.